NRS Preliminary Results 2010

Preliminary estimates of Blue Tit nesting success can be
published early thanks to the efforts of nest recorders

The Nest Record Scheme (NRS) is an annual survey designed to collect information on the timing of breeding and the productivity of nesting attempts. Each spring and summer, over 500 volunteer nest recorders visit their local patches and monitor the progress of more than 30,000 nests of a huge range of bird species. Using the data they collect, the BTO can produce trends in laying dates, clutch and brood sizes and failure rates.

The trends in breeding success over the last four decades are published each year in the BirdTrends Report, but it currently takes almost 12 months to collate and analyse the full dataset. In 2010, we asked nest recorders to make a special effort to send their data in as early as possible so that we could produce a preliminary report on the season by the end of the year. They rose to the challenge and by the end of October, over 10,500 records had been submitted, over double the number we usually receive over this time.

This fantastic effort allowed us to produce preliminary estimates of breeding success for the seven most commonly monitored species: Barn Owl, Swallow, Blackbird, Pied Flycatcher, Blue Tit, Great Tit and Tree Sparrow (Table 1).

Species Clutch size Brood size Egg stage failure rate Chick stage failure rate Fledglings per breeding attempt
Barn Owl - -10.8 - - -
Swallow ns ns ns ns ns
Blackbird ns ns ns ns ns
Pied Flycatcher ns +2.8 ns -9.3 +14.7
Blue Tit +3.5 +8.6 ns -9.6 +17.6
Great Tit +7.6 +5.6 -1.9 ns +12.5
Tree Sparrow ns ns ns ns ns

Table 1. Percentage change in breeding success in 2010 relative to the average for the previous five years. 'ns' indicates a statistically non–significant difference. Insufficient data for Barn Owl were available to estimate clutch size, failure rates or FPBA.

Last year was clearly a productive one for our nest box users, with the number of fledglings produced per breeding attempt (FPBA) significantly greater than average for Blue Tit, Great Tit and Pied Flycatcher (figure 1). Blue Tits and Great Tits both laid large clutches, which hatched to produce large broods, and failure rates at the egg and nestling stages respectively were low (Table 1). Nestling failure rates were also low for Pied Flycatcher, all three species possibly benefiting from the driest April-May period for 20 years.

Figure 1. Fledglings per breeding attempt 1965 to 2010 for Blue Tit (left), Pied Flycatcher (middle) and Great Tit (right).

In contrast, Barn Owls had a disappointing season in 2010, with mean brood sizes falling to their lowest level for a decade (Figure 2). The extremely cold winter of 2009/10 may have left females in poor condition at the start of the breeding season, reducing the amount of energy available to invest in egg production, leading to below average clucth sizes. The harsh winter weather conditions may also have affected vole numbers later in the season, although snow cover can actually be beneficial to small mammals as it shields them from predators.

Barn Owl chicks. Photo by Colin Shawyer

Figure 2. Barn Owl brood sizes in 2010 (red dot) were the second lowest since 1985 

Swallow, Blackbird and Tree Sparrow all appeared to experience a very average breeding season, with clutch sizes, brood sizes and failure rates all similar to those recorded over the preceding five years.

There are several more species for which we are on the verge of being able to produce preliminary estimates, including Song Thrush, Chaffinch and Robin. Hopefully the results presented here will encourage other nest recorders to try and submit their data earlier so that we can include these in next year’s figures. If you'd like to become a nest recorder yourself, download your NRS Quickstart Guide or nrs [at] bto.org (email us).

Acknowledgements

We’re extremely grateful to all those nest recorders taking part in the NRS, both for collecting the data and making the additional effort to submit it quickly, and for the support given under the JNCC/BTO partnership that the JNCC undertakes on behalf of the Country Agencies.